Major seed crops grown in seasonal climates are often winter crops, that is, crops are sown in the autumn to germinate and overwinter vegetatively and then flower and set seed in the spring and early summer.
Such plants achieve most of their growth while temperatures can be far below the optimum growth temperature for that species. As such, crop plant growth in well resourced fields is often limited simply by the temperature of the growing environment. In a nursery situation where plants are grown in glasshouses, winter heating to an ideal growth temperature is commonly a significant contributor to the cost of plant production. Hence in both situations there is interest in understanding how the relationship between environmental temperature and plant growth rate is established, and whether this can be modified either through conventional or fast-track breeding or biotechnological means. It has often been assumed that plants grow slower under cool conditions simply because of the general affect of low temperature on biochemical reaction rates within plant cells. However, ecologists and nursery growers have long realized that different species show different temperature optima for growth, suggesting that the relationship between environmental temperature and growth rate is under genetic control. Understanding these mechanisms furthers the possibility of targeted modification of these traits. In a recent paper [1] we identified that the SPATULA (SPT) gene is responsible for decreasing
Arabidopsis thaliana growth rates in response to low ambient temperatures, particularly during the early part of the day. As spt mutants show no increase in freezing sensitivity, deletion of this gene could be a viable way to increase crop yields in temperate areas at cooler times of year.
Growth and survival at low temperatures
Arabidopsis thaliana and other plants are able to tolerate temperature below 0 ºC, and this freezing tolerance can be increased by acclimating plants at very low non-freezing temperatures. The CBF/DREB genes, which are induced at the transcriptional level as temperature decreases, are responsible for much of the cold acclimation response and are also involved in drought and salinity resistance. Constitutively over-expressing these genes in Arabidopsis leads to increased freezing tolerance but stunted growth at normal temperatures, showing that in this case there is a metabolic trade-off between growth and stress tolerance [2]. Pathways similar to the Arabidopsis CBF cold acclimation pathway are found in the related crop species Brassica napus, and also in chilling-sensitive tomato. Overexpression of Arabidopsis CBFs in these plants promotes freezing and chilling tolerance, respectively, but does not induce freezing tolerance in the tomato. This work shows that in some species there is overlap between the molecular mechanisms controlling tolerance to freezing and the control of growth. Because high levels of CBFs lead to growth repression, deletion of CBFs might increase growth at lower temperature. However, this effect is likely to include the severe penalty of an increase in chilling and freezing sensitivity of such plants. SPT is a basic helix-loop-helix (bHLH) transcrip- tion factor, similar to Phytochrome Interacting Factors (PIFs) that are involved in growth responses to light in particular [3]. Certain PIFs are implicated in growth responses to temperature; specifically, PIF4 is involved in high temperature elongation responses [4]. SPATULA was originally identified as a seed pod morphology gene, taking its name from the broadened end of the mutant silique. We observed that at lower ambient temperature,
Arabidopsis thaliana plants lacking functional SPT protein grow more quickly than wildtype plants. At normal temperature these mutants show few differences in growth relative to wildtype. This phenotype was observed in different spt loss-of-function mutants in two different ecotypes. In addition, plants with the synthetic construct 35S::SPT-HA, which constitutively overexpress SPT, show a strong growth reduction at low ambient temperature. Again, at normal temperature, growth of these SPT-overexpressing plants is similar to wildtype. A freezing-tolerance experiment reveals that spatula mutants have no increase in sensitivity to freezing temperature. These experiments tested whether the plants could acclimate to cold temperature by keeping them at 4 ºC for a few days before subjecting them to freezing temperature, as well as testing basal freezing tolerance by transferring the plants directly from ambient to freezing temperature. In both cases, the spt mutants had the same survival rates as the wildtype plants, showing that they retain their ability to sense and respond to extreme, potentially harmful, low temperature. In this case there appears to be no compromise between stress tolerance and growth rates. SPT could therefore be considered a good candidate gene for deletion, mutation, or modification in order to improve crop yields for crops grown in spring and autumn in temperate regions.
Thermoperiodism
It has been known for a while that for certain plants a higher day temperature and lower night temperature can increase growth and in some species cause morphological changes such as internode elongation. This effect is used for growth control in production of ornamental and crop plants propagated in a green-house. Higher temperatures during the day promote stem elongation while a low daytime temperature and high night-time temperature can produce compact flower plants and vegetable seedlings with short internodes, without a delay in production time [5]. These temperature treatments are widely used as an alternative to chemical growth control.
In Arabidopsis grown at 15 ºC, which is for them a low ambient temperature, giving a daily 8- or 4-hour pulse of warmth only causes an increase in growth if this warmth is applied during the light period. So it appears
that daytime temperature is particularly important for these kinds of growth responses. spt mutants also show increased growth in response to a daily 8- or 4-hour pulse of warmth during the light period, particularly when these pulses occur in the late part of the light period. SPT appears to repress growth in response to cool ambient temperatures, specifically when this temperature is experienced by the plant during the earlier part of the light period. A four-hour warm treatment in the morning promotes wildtype growth equal to that of the spt mutant but does not affect spt mutant growth. This shows that warm temperatures and loss of SPT cause an equivalent and epistatic loss of growth repression at this time of day. In addition, spt mutant plants increase growth in response to four- or eight-hour warm periods later in the light period. This is likely because in these treatments, without SPT, there is a lack of growth repression in response to cool morning temperatures, combined with growth promotion by warmth through other factors later in the day. Plants with mutations in SPT could therefore be used in commercial greenhouses to reduce heating costs.
Gibberellin
One further interesting feature of growth inhibition by SPT is that it is not linked to the Gibberellin (GA) pathway. Gibberellin is a plant hormone that promotes growth, seed germination, and fruit set, and it is used commercially to increase yields of crops such as seedless grapes, apples, and cane sugar. The Gibberellin pathway has other roles in increasing crop yields, for example, the dwarfed wheat varieties that were central to the Green Revolution of the 1960s and 1970s. These varieties have a mutation blocking the degradation of a growth-repressing DELLA protein, which is normally degraded in response to Gibberellin. The plants are thus shorter, have increased grain yield at the expense of straw biomass, and are more resistant to lodging [6]. Thus alteration of the new SPATULA-dependent pathway would not be expected to reverse gains in performance achieved through dwarfing, and because SPT acts completely independently of GA in regulating growth, the two pathways could both be used in the same plants to promote growth.
Conservation of SPATULA in angioseperms
Arabidopsis thaliana is in the same family as the oilseed crop plant Brassica napus, sometimes called Canola, making this the most obvious crop plant to investigate for commercial utilization of this spatula phenotype. The SPT gene can be readily identified in angiosperm species with large sequence coverage, and even in poplar, suggesting wide conservation among commercially important plants.
Summary
The Arabiopsis thaliana protein SPATULA controls growth in response to cool ambient temperatures, especially when these occur during the early part of the light period. Plants lacking SPATULA grow more than wildtype under conditions of low ambient temperature but show no differences in tolerance to freezing, making it a good candidate gene for improving growth during autumn and early spring in temperate regions. As spatula mutants appear to specifically lack the ability to restrict growth in response to cool temperature in the early part of the day, such mutants could be used in commercial greenhouses to reduce heating costs. In control of growth, SPATULA action does not seem to be linked to the Gibberellin pathway, allowing genetically engineered plants that utilize both pathways to improve yields. All of these features make SPATULA a strong candidate gene for improving crop yields, particularly at cooler times of year. Learning more about the molecular genetic pathway controlled by SPT should lead to the discovery of further targets for crop improvement.
References
1. Sidaway-Lee K, et al. SPATULA Links Daytime Temperature and Plant Growth Rate. Current Biology 20(16), 1493-1497 (2010)
2. Kasuga M, et al. Improving plant drought, salt, and freezing tolerance by gene transfer of a single stress-inducible transcription factor. Nature Biotechnology
17, 287-291 (1999)
3. Leivar P & Quail PH. PIFs: pivotal components in a cellular signaling hub. Trends in Plant Science, In Press. (2010)
4. Koini MA, et al. High temperature-mediated adaptations in plant architecture require the bHLH transcription factor PIF4. Curr. Biol. 19, 408-413 (2009)
5. Myster J & Moe R. Effect of diurnal temperature alternations on plant morphology in some greenhouse crops—a mini review. Scientia Horticulturae 62(4), 205-215 (1995)
6. Peng J. et al. ‘Green revolution’ genes encode mutant gibberellin response modulators. Nature. 400, 256-261 (1999)
